Tensioning device

ABSTRACT

There is provided a tensioning device for pretensioning mooring lines in accordance with the tension-in-a-bight principle. The tensioning device is located at the bight of the mooring lines and includes a link catching device comprising inclined land surfaces and a catching tooth at the lower end of the land surfaces, which catching device catches a link, the caught link, of link chain during heaving up on a riser chain section so enabling the pretensioning to be achieved in the mooring line. Slots are provided in the tensioning device adjacent the land surfaces such that the downline link threading the caught link and the following downline link each have a significant degree of pivotal freedom to preclude bending moment loading in these links during heaving up on the riser line during tensioning.

The present invention relates to a tensioning device for pretensioningspread mooring line systems having spaced opposed anchor members lodgedin a mooring bed and anchor cable means connected to each anchor forattachment to a vessel or other floating marine structure to be moored.It is to be understood that the term "cable" covers link chain cable inaddition to wire cable and, indeed, all line constructions equivalent tosuch wire and chain cables.

It is present practice to pretension the cables of such mooring linesystems to set the ground-engaging portions of the cables into the soilwhen the anchor members comprise driven piles and, additionally, tocause penetration of the anchor members themselves when the anchormembers comprise drag embedment fluke anchors. A common method ofpretensioning involves simply hauling in on opposed cables using a winchmounted on a floating vessel. This has the disadvantage of requiring alarge and costly high-powered winch when large anchor members aredeployed. This disadvantage can be mitigated substantially by adopting amethod involving the well-known tension-in-a-bight purchase principlewherein an increment of tension applied to a first cable attached atright angles to the middle of a stretched second cable deflects it andso induces an increment of tension in the second cable which byresolution of forces, may be several times greater than that applied tothe first cable. The component of tension at right angles to the firstcable that is induced in the second cable is equal to the tension in thefirst cable divided by twice the tangent of the angular deflection ofthe second cable at the attachment point. Thus, the power and cost ofthe tensioning winch required to pretension a mooring line system may bereduced greatly by pulling vertically on a riser cable attached at ajoining point between two lengths of anchor cables stretched outhorizontally over a mooring bed between two anchor members to benefitfrom the multiplication of tension obtained from the tension-in-a-bightpurchase principle.

It is an object of the present invention to provide an improvedtensioning device for pretensioning mooring line systems when using thetension-in-a-bight purchase principle.

According to the present invention there is provided a tensioning devicefor use in pre-tensioning at least first and second cables of a mooringline system when the device is attached thereto, at least one of thecables including link chain cable, the tensioning device comprising abody member including at least first and second cable holding means, andpassage means located between the cable holding means to enable the onecable including link chain to pass through the body member to define ariser cable portion, at least one of the cable holding means comprisingreleasable catch means for engaging the link chain cable, the releasablecatch means being arranged such that a link, referred to as the caughtlink, of the link chain cable extending through the passage means iscaught by the releasable catch means when the riser portion is tensionedand released from the releasable catch means when the riser portion isrelaxed, the body member including structure defining slot and clearancemeans in the body member whereby when the caught link is engaged by thereleasable catch means during chain tensioning a downline chain linkthreading the caught link and a following link can move into the slotand clearance means, and the downline chain link can swing to form anangle in the range 0° to 27° with a plane containing a line connectingthe first and second cable holding means, defined as the cable holdingmeans connecting line, without a portion of the body member contactingthe downline chain link.

Preferably the body member includes land surface means adapted tosupport only said caught link of link chain adjacent said releasablecatch means, and preferably said land surface means support the caughtlink in a plane inclined at an acute angle to the horizontal when thecaught link (L1) engaged by the catch means during chain tensioning, theacute angle facing the axis of the riser portion.

Preferably means are provided for the attachment of a pendant line tothe body whereby the catch means are released on heaving up on thependant line.

Embodiments of the present invention will now be described by way ofexample with reference to the accompanying drawings wherein:

FIG. 1 is a sectional side elevation of a tensioning device throughsection M--M in FIG. 3 for tensioning anchor chains, in accordance withthe present invention;

FIG. 2 is a plan view of the device in FIG. 1;

FIGS. 3 and 4 are end views looking in the directions of A and Brespectively;

FIG. 5 is a view in the direction C, i.e. onto land surfaces of thedevice for a caught-link;

FIGS. 6A-6H show a series of sectional side elevational views of thedevice illustrating various operational stages; and

FIG. 7 shows an elevation, in part section, of an anchor chaintensioning device in accordance with another embodiment of the presentinvention.

Referring especially to FIGS. 1 to 5, a mooring line tensioning device 1serves for tensioning opposed anchor chains 18, 19 (see FIG. 6A) to theouter ends of which chains individual anchors (not shown) are attached,the device 1 comprising a body member 2 generally of block form havingside walls 3 and a transverse portion 4. The transverse portion 4includes releasable link catch means 6A in the form of a clutch deviceto catch a link of the chain defined by chain portion 18 and a riserchain portion 17, while aligned holes 28 in the side walls 3 spaced fromcatch means 6A serve to receive a shackle pin for pivotally securing theanchor chain 19 to the device 1. A passage means 26 for the chain riserportion 16 is located between the catch means 6A and the shacklereceiving holes 28.

A connecting straight line K--K joins the two chain holding meansdefined by catch means 6A and shackle holes 28; more specifically lineK--K passes through the centre of holes 28 and through the centre of thecrown CI of the caught link LI caught by the catch means 6A, said crownCI being engaged by the catch means 6A. In the FIG. 7 embodiment afurther releasable catch means 6B is present replacing shackle holes 28)and located oppositely to the catch means 6A, and in this case theconnecting line K--K passes through the centres of the caught linkcrowns CI associated with the respective catch means 6A, 6B. Theconnecting line K--K defines the term "cable holding means connectingline" and the line is horizontal in FIGS. 1 and 6A.

The catch means 6A is in the form of a tooth 8 having a front surface 8Aand a back surface formed by a clearance 8B in the body member 2, andland surfaces 7 are provided before the tooth 8 to support a link LI(referred to as the caught link) the land surfaces 7 forming a pocketformation with portions of the side walls 3 and tooth 8 (see FIG. 5).The catch means 6A lock onto the caught link LI with the tooth frontsurface 8A bearing against the adjacent crown of the caught link LI. Aslot is provided between the surfaces 7 for passage of chain linkspositioned in a vertical plane (referred to as vertical links), thecaught link LI threading vertical links.

The land surfaces 7 have support surface portions 7A which support thecaught link LI adjacent the crown C2 remote from the tooth 8; and thesurfaces 7 further include recesses 7B to accommodate weld joins in thechain links and so avoid the caught link being bent by pressure onprotruding weld metal. For effective operation of the device 1 the landsurfaces 7 are inclined at an acute angle to the cable holding meansconnecting line K--K, and it has been found that the preferred acuteangle lies in the range 30° to 70° with 35° to 45° preferred. In thisexample the acute angle is about 40°.

The body 2 is configured, i.e. provided with a deep clearance slot 9B,so that the down-line vertical link L2 threading caught link L1 canswing to form an angle of 27° or less with the line K--K without aportion of the body member 2 contacting link L2: in this example thelink L2 can in fact swing to be aligned with line K--K i.e. form a zeroangle. Further, the body clearance 8B (i.e. tooth back surface) is suchthat with the link L1 caught by catch means 6A, link L4 threadingvertical link L3 (as shown in FIG. 1) can swing to the same angle aslink L3 without contacting a portion of the body member 2 above chainlink L4. A further slot 9A at the upline end, enables vertical link L3to swing freely to the upright position as shown, the links L3 formingpart of the riser portion 17 of the anchor chain 18.

The passage 26 is in the form of a slotted guide means which serves toorientate chain passing therethrough so that a non-vertical link ispositioned for ready engagement by the catch means 6A and support bysurfaces 7 i.e. to serve as a caught link L1. As can be seen in the planview shown in FIG. 2 the slotted guide means have a cruciform crosssection. The body member 2 further includes a slot 30 to enable thedevice 1 to be fitted onto the anchor chain 17, 18. In this example, thetooth front surface 8A is preferably inclined at an angle in the rangeof 90° to 120° to the surfaces 7, and more especially in the range 96°to 114°, to enable the caught link L1 to swing easily away from thecatch means 6A when the chain 17, 18 is relaxed as will be explainedlater.

To enable the catch means 6A to be freed from the caught link L1, a hole12 is provided for a shackle 13A of a pendant linke 13, and the hole 12is positioned to provide an appropriate lever arm for the pendant line13 whereby the catch means 6A is tripped out of catching engagement withthe caught link L1 on heaving up on the pendant line 13. In particular,the hole 12 is spaced appropriately from the surface portion 7A toprovide the desired lever arm. Ears 31 are provided on the body member 2below the holes 28 to prevent excessive downward swinging of the shackleattaching chain 19 but in practice excessive downward swinging of chain19 is unlikely to occur. Further, stop elements 32 on the body member 2serve to limit the downward swing of the pendant shackle 13A.

The transverse portion 4 is cut away at 4A to enable the links L3 and L5to swing adequately to the right (as shown in FIG. 1) to enable a vesselto ride at long scope when pulling through the anchor chain 19 with theriser portion 17 fairly substantially inclined from the verticalsubsequent to completion of the tensioning operation.

To tension mooring chains 18, 19 utilising the device 1, a first anchoris first laid on the sea bed, say that connected to chain 17, 18, andthe device 1 can then be fitted to this chain 17/18. With the chain 19(complete with second anchor) now fitted to the device 1 by means ofholes 28, the device 1 can be lowered down riser chain 17 utilisingpendant line 13 to enable positioning of the second anchor on the seabed and stretching out of chains 18/19--the situation is now essentiallyas shown in FIG. 6F with the link LI in contact with land surfaces 7.

To effect the tensioning operation the riser chain 17 is heavedvertically and this vertical heaving on the riser chain 17 (FIG. 6A)keeps the caught link L1 firmly locked against the land surface 7 andagainst the tooth 8 and induces a horizontal component of tension ineach of chain legs 18, 19 equal to the vertical riser tension divided bytwice the tangent of the angle of inclination of link L2 to thehorizontal. This horizontal component of tension, reaching as high as 6times the riser chain tension for 4.76° inclination of link L2 to thehorizontal (line K--K), is available for setting the ends of chain legs18, 19 and anchors attached thereto into the sea bed. A veryconsiderable mechanical advantage is thus present at low angles ofinclination of link L2 which decreases below unity when the angle ofinclination of link L2 is as high as 26.6°.

As device 1 is raised progressively higher above the sea bed, theinduced horizontal tensions in chain legs 18, 19 cause movement of theanchors or burial of the chain ends which relax the chains and limit thehorizontal tensions to a maximum peak value which, when reached, denotescompletion of the first heave of the tensioning operation.

On completion of the first heave, paying out the riser chain 17 (FIG.6B) now reduces the tension in all three chain legs 17, 18, 19. As thetension decreases each of the lower chain legs 18, 19 sags to aprogressively larger angle to the horizontal whilst the tensionerrotates about a pivotal curved surface at the upper edge of the landsurface 7 supporting the caught link. This causes the caught link LI,the next lower link L2 threaded through it, and the next again link L4to rotate relative to the tensioner I until the catch means 6A aredisengaged completely from the chain links.

Further paying out of the riser chain 17 (FIG. 6C) causes theinclination of the caught link L1 to increase until the tensioner 1slides downwards along it, gathering momentum on the way.

In FIG. 6D downwards movement of the tensioner 1 has brought the caughtlink L1 in line with the vertical riser leg of chain 17.

The tensioner 1 then falls (FIG. 6E) further until the pivotal surfacemakes contact with the second link L4 below the caught link whereuponthe sequence of FIGS. 6A to 6D is repeated.

In the above manner, the tensioner 1 cascades down the chain to the seabottom (FIG. 6F) where its weight produces a small tension in the chainsenabling a fresh link to be caught against land surface 7 and the catchmeans 6A. Heaving can now be commenced again to establish a new peakhorizontal component of tension obtained in each of the lower legs ofthe chains with further movement of the anchors or embedment of thechain ends occurring. These heaving cycles are repeated until the peakhorizontal component of tension in chains 18, 19 achieved per heaveincreases to equal the tension desired.

When the heaving cycles of FIGS. 6A to 6F have been completed and thetensioner has returned to the position shown in FIG. 6F, heaving up onthe pendant line 13 while keeping the riser chain 17 slack rotates thetensioner on the caught link L1 and disengages the catch means 6A fromthe crown C1 of the caught link L1 (shown in FIG. 6G).

Further heaving on the pendant line 13 (FIG. 6H) hauls the tensioner 1up the slack riser chain for recovery and removal from the riser chain17.

If the device 1 utilises land surfaces 7 which are set at a greaterangle to the line K--K, in particular greater than 50°, and/or where alower frictional factor is present on a pivotal curved surface at theupper end of land surface 7 the operation of the device may be somewhatdifferent. Thus, instead of the swinging motion between the device 1 andthe links L1, L2 and L4 as shown in FIG. 6B, in this case the device maysimply slide over the link L1 to bring the tooth back surface 8B intocontact with the upstream crown of the link L4. For this situation, theback surface 8B requires to be adapted so that it can push aside thelink L4 to enable continued downward movement of the device 1. Thus inFIG. 1, for this requirement, the back surface 8B has an outer portion Iadjacent the crown 8C of tooth 8 which is inclined with a downwardopening angle to land surfaces 7 in the range 40° to 80°. A preferredrange is 47° to 57°; the latter value being specifically used in FIG. 1.The surface 8B has a further portion II following portion I which is atan angle to portion I to allow the required swinging motion of link L4.

Essentially the above preloading operation is achieved by utilising theknown tension-in-a-bight purchase technique, the resisting horizontalcomponent of tension in the chain sections being greater than in theriser section 17 by virtue of the previously mentioned reciprocaltangent relationship. The full desired mooring preload is achieved by asuitable number of the above preloading stages, and the device 1 can beretrieved ultimately from the cable sections by the pendant line 13 whenpreloading is complete.

By virtue of the slots 9A, 9B in the portion 4, when the chain 17, 18 istaut with link L1 caught in the catch means 6A, there is no substantialchain contact with other portions of the device 1 so that the tensioningdevice 1 engages the chain 17/18 essentially only on the caught link L1which is suitably supported to avoid the link being bent. Thisarrangement mitigates against excessive stress loading occuring in linksL2, L3 and L4 and so avoids the chain being damaged by the tensioningdevice 1 during pretensioning.

The modified form of tensioning device 41 shown in FIG. 7, functions tograb on two chains i.e. the device is double sided, and includes afurther chain clutch device 6B replacing the shackle bolt holes 28 ofthe FIG. 1 device, the clutch device 6B being the mirror image of theclutch device 6A. The device 41 functions similarly to the tensioningdevice 1 of FIG. 1 but in this case there are two riser chain portions,one connecting with anchor chain 18 and the other with anchor chain 19.Holes 12 for pendant lines 13 are again provided and the device 41 canbe removed when pretensioning is complete simply by hauling up on thependant lines 13. The device 41 is formed by a pair of side plates 3joined by transverse spacing portions 4 which provide the clutch devices6A, 6B as before. The connecting line K--K between the catch or clutchdevices 6A, 6B is shown in FIG. 7.

More than two anchor cables could be tensioned using either the abovetensioning device 1 (or 41) suitably modified or by setting uptensioning apparatus using a plurality of the tensioning devices 1 (41).

I claim:
 1. A tensioning device for use in pretensioning at least firstand second cables of a mooring line system when the device is attachedthereto, at least one of said cables including link chain cable, saidtensioning device comprising:a body member including at least first andsecond cable holding means, and passage means located between said cableholding means for enabling said one cable including link chain to passthrough the body member to define a riser portion, said first and secondcable holding means defining a cable holding means connecting line, andsaid riser portion being selectively movable between a tensionedcondition and a relaxed condition, at least one of said cable holdingmeans comprising releasable catch means for engaging said link chain,said releasable catch means being positioned to catch a link of saidlink chain when the riser portion is tensioned and to release said linkwhen the riser portion is relaxed, said link caught by said catch meansbeing defined as the caught link, the caught link having a downlinechain link threading the caught link, and a following link threading thedownline chain link, said body member further including slot andclearance means therein for receiving the downline chain link when thecaught link is engaged by the releasable catch means during chaintensioning which permits the downline chain link to occupy withoutobstruction an axial line which forms an angle in the range 0° to 27°with a plane containing said cable holding means connecting line.
 2. Atensioning device as claimed in claim 1, wherein the body memberincludes land surface means for supporting only said caught link of thelink chain adjacent said releasable catch means.
 3. A tensioning deviceas claimed in claim 2, wherein said land surface means supports thecaught link in a plane inclined at an acute angle to the horizontal whenthe caught link is engaged by the releasable catch means during chaintensioning, the acute angle facing the axis of the riser portion.
 4. Atensioning device as claimed in claim 3, wherein said acute angle ofinclination of said plane of support for the caught link lies in therange 30° to 70°.
 5. A tensioning device as claimed in claim 2, whereinsaid caught link has a crown which is remote from the releasable catchmeans, and wherein said land surface means comprises a support surfacewhich supports the caught link closely adjacent the caught link crown.6. A tensioning device as claimed in claim 1, wherein the releasablecatch means comprises a chain clutch device.
 7. A tensioning device asclaimed in claim 6, wherein the caught link has a crown, and wherein thechain clutch device includes tooth means for engaging and retaining saidcrown of the caught link.
 8. A tensioning device as claimed in claim 7,wherein said tooth means has an outer surface and a crown, and whereinthe outer surface of the tooth means has a portion adjacent to the toothcrown which is inclined at an acute angle to the plane of support forthe caught link, whereby chain links bearing on said portion of saidouter surface slide clear of the tooth means when the tensioning devicefalls from the chain on relaxation of the chain tension.
 9. A tensioningdevice as claimed in claim 8, wherein said inclination of said portionof said outer surface of the tooth means to the plane of support for thecaught link is in the range 40° to 85°.
 10. A tensioning device asclaimed in claim 7, wherein the inner surface of the tooth meansengaging the caught link crown lies at an angle to the plane of supportfor the caught link in the range 90° to 120°.
 11. A tensioning device asclaimed in claim 1, wherein said passage means has a portion in the formof slotted guide means for controlling the orientation of link chainpassing therethrough so that a link of the link chain is presented forready engagement by the releasable catch means.
 12. A tensioning deviceas claimed in claim 11, wherein said slotted guide means has a cruciformcross-section.
 13. A tensioning device as claimed in claim 1, whereinthe body member includes means for attaching a pendant line positionedsuch that heaving up on the pendant line trips said releasable catchmeans out of engagement with the caught link to permit retrieval of thetensioning device.
 14. A tensioning device as claimed in claim 1,wherein one of the cable holding means comprises said releasable catchmeans while the other cable holding means comprises means for pivotallysecuring a cable to the body member.
 15. A tensioning device as claimedin claim 1, wherein at least two of the cable holding means comprisereleasable catch means.
 16. A tensioning device as claimed in claim 14,wherein means for attaching a pendant line are located on the bodymember spaced from the extremity of said land surface means remote fromsaid releasable catch means, whereby a lever arm is established whichpromotes tripping of the releasable catch means out of engagement withthe caught link when heaving up on a pendant line attached to saidpendant line attaching means.
 17. A tensioning device as claimed inclaim 1, wherein more than two releasable catch means are present, eachfor engagement with a respective link chain cable.
 18. A tensioningdevice as claimed in claim 15, wherein means for attaching a pendantline are located in an axis of symmetry of the body member.